1. Field of the Invention
The present invention relates to a thermal transfer film comprising a coloring layer formed on a substrate film via an intermediate layer, and more particularly relates to a thermal transfer film which can give a clear printing without lack of impression to a paper to be printed and allows reliable coating of an intermediate layer and a coloring layer on a substrate film and has secret leakage preventing properties, and an image forming method using the same.
2. Description of the Related Art
Conventionally, thermal transfer films comprising a coloring layer comprising a thermally fusible ink formed on one side of a substrate film have been used as thermal transfer recording media used for thermal transfer printers, facsimiles and the like.
Conventional thermal transfer films have substrate films made of about 10 to 20 μm thick papers such as condenser paper and paraffin paper or about 3 to 20 μm thick plastic films such as polyester and cellophane and coloring layers obtained by coating on the substrate film thermally fusible inks which are mixtures of binders, colorants such as pigments and dyes, and additives such as melting point-lowering agents and plasticizers as occasion demands. Some thermal transfer films have intermediate layers adjusted to melt by the energy for printing between the substrate films and coloring layers.
The substrate films are heated and pressed in the predetermined areas by thermal heads from behind to melt the coloring layers corresponding to the printing areas to transfer the same onto transfer receiving materials for printing.
However, when the conventional thermal transfer films having intermediate layers and thermally fusible coloring layers formed on substrate films are used for printing, there have been problems that letters and fine lines are blurred by lack of impression to give the printed materials a patchy appearance and that there is a large noise emitted when the thermal transfer films are separated from transfer receiving materials. In order to print without lack of impression on coarse papers with a Beck smoothness of 50 seconds or lower, it is necessary to transfer all the coloring layer without occurrence of lack of impression to the printed materials (without being left on the intermediate layer) in the areas to which energy is applied according to the pixel by a means such as a thermal head to a transfer receiving paper. It is effective to separate the transfer receiving material from the thermal transfer film when the intermediate layer of the thermal transfer film, which has a coloring layer via the intermediate layer on the substrate film, is melted and flowable, and therefore in a liquid state in order to transfer all the necessary coloring layers to the transfer receiving paper. However, there is a time interval between the time when the transfer receiving material and the thermal transfer film are superimposed and the printing energy is applied to the thermal transfer film and the time when the thermal transfer film is separated from the transfer receiving material in machines generally used such as facsimiles using thermal transfer films. There arises a disadvantage in that the intermediate layer is cooled and solidified or decreases in flowability if not solidified in the time interval even when the intermediate layer is adjusted to melt by the printing energy.
Incidentally, materials having so-called supercooling properties, which have freezing points 10° C. or lower than their melting points, are known in various literatures. Techniques about thermal transfer films having coloring layers on the substrate films via intermediate layers comprising various materials having supercooling properties are known. For example, such techniques are disclosed in Japanese Patent Application Laid-Open Nos. 61-235189, 61-286195, 62-9991, 62-82084, 63-302090, 3-246094 and others. On the other hand, it is well known that polycaprolactone-based resins have supercooling properties in various literatures. Techniques about thermal transfer films having coloring layers containing the polycaprolactone-based resin formed on the substrate films are well known. For example, such techniques are disclosed in Japanese Patent Application Laid-Open Nos. 59-230795, 60-122194, 60-122195, 61-185492, 62-59089, 5-32073 and others.
Furthermore, techniques about thermal transfer films having coloring layers via intermediate layers containing the polycaprolactone-based resin formed on the substrate films are well known. For example, Japanese Patent Application Laid-Open No. 60-165291 discloses that the polycaprolactone-based resin is used in an intermediate layer for the purpose of multiple printing and Japanese Patent Application Laid-Open No. 7-232483 discloses that polycaprolactone with a molecular weight of 10,000 or less is used in a primer layer for the purpose of facilitating high-speed printing and smooth printing in a high temperature atmosphere.
However, the thermal transfer films employing the intermediate layers according to these techniques still have the disadvantage in that letters and fine lines are blurred by lack of impression to give the printed materials a patchy appearance. What is worse, the thermal transfer films have a disadvantage in that when the ink for forming the intermediate layer is coated on the substrate film, the intermediate layer material stays melted by the heat for drying for a while even after the intermediate layer ink has been heated and dried, which undesirably causes adhesion between the substrate film side of the thermal transfer film wound after the coating and the intermediate layer side. Moreover, when a coloring layer is coated on the substrate film having an intermediate layer formed thereon, the hot-melt coating method, which facilitates the low-cost coating because no solvent is needed, has a disadvantage in that polycaprolactone present in the intermediate layer is melted and becomes fluid by the heat of the heated and melted coloring layer ink, which prohibits the coloring layer ink from being coated with a good surface quality.